Unmask legacy interrupts on Marvell PCIE controller

This patch fixes a bug introduced with commit:
r294510 "Remove an extra '!' found by clang 3.8."

'!' was removed without inverting the logic, which
broke PCIe legacy interrupts operation for Marvell
controllers.

Submitted by: Michal Mazur <mkm@semihalf.com>
Obtained from: Semihalf
Sponsored by: Netgate